This invention relates to a fuel feeding device for a heating unit, especially for vehicles of the type preferably comprising a combustion air fan, the feed line of which fans terminates in a combustion chamber and contains a constriction in the manner of a venturi tube, a fuel line ending in the feed line.
In some conventional vehicle heating units, an electromagnetically operated diaphragm pump is provided, the intake chamber of which is connected, via a first check valve, with an intake pipe in communication with a fuel tank and, via a second, oppositely effective check valve, with a feed line leading to a metering device, by means of which a fuel is introduced into the air feed line in dependence on the amount of air conveyed by the combustion air fan. This metering device consists of a differential pressure box with a diaphragm separating two chambers from each other, wherein the first chamber is connected via a shutoff valve actuated by the diaphragm with the feed line of the fuel pump, and furthermore, with the air feed line in the zone of the venturi tube, whereas the second chamber is in communication with the air feed line outside of the venturi tube. The pressure difference between the two chambers forms a yardstick for the amount of air conveyed by the combustion air fan, and the diaphragm is moved thereby in such a way that a control valve connected to the diaphragm allows an amount of fuel to pass which is required for the formation of the desired fuel-air ratio. Besides, an electromagnetically operable shutoff valve is provided in most cases in the intake line of the fuel pump which prevents fuel from passing from the fuel tank to the air feed line when the device is turned off. This conventional fuel feeding system thus requires a plurality of individual units and thus is relatively complicated and expensive. The series connection of several throttle points in the feed line of the fuel pump, constituted by the check valve in the feed line of the pump, the control valve actuated by the diaphragm of the metering device, and the fuel nozzle, furthermore impairs the accuracy of the metering action of the fuel pump.
The invention is based, at least in part, on the problem of providing a fuel feeding device for a heating unit of the aforementioned type which is distinguished by a substantially simpler structure and which makes it possible to effect an accurate metering of the amount of fuel, extensively independently of the effects of temperature or air pressure.
This problem is solved, in accordance with a preferred embodiment of the invention, by providing that the fuel feeding device comprises a diaphragm pump, the diaphragm of which separates two chambers from each other, of which the first chamber is in communication, via a valve, with the fuel line and, via a fuel nozzle as well as a check valve, with the air feed line in the zone of the venturi tube, and the second chamber is connected to the air feed line at a position spaced from the venturi tube; and wherein an electromagnet is provided which, upon excitation, moves the diaphragm along the lines of a suction stroke with reference to the first chamber, a switch being disposed in the circuit of the coil of the electromagnet which is closed at the end of the delivery stroke of the diaphragm and opened at the end of the suction stroke.
A suggestion advanced by this invention, therefore, combines the fuel pump and the metering device in one device, wherein the suction stroke of the pump is effected by the electromagnet, whereas the delivery stroke is effected by the pressure difference in the air feed line between the venturi tube and a point outside of the venturi tube and thus is automatically dependent on the amount of air conveyed by the combustion air fan. This suggestion of this invention does not only result in a significant simplification from a constructional viewpoint, it also constitutes a functional improvement over the conventional arrangement, since only a check valve and a fuel nozzle are disposed in the feed line of the pump, whereas the control valve required in the above-discussed known device and actuated by the diaphragm of the metering means has been eliminated.
The valve in the fuel line can be a simple check valve. However, preferably, this valve is a shutoff valve operated by the electromagnet, which is opened when the electromagnet is energized. This, in turn, yields a considerable structural simplification as compared to the conventional devices with a separate solenoid valve.
Preferably, the electromagnet comprises an armature connected, on the one hand, to the diaphragm and, on the other hand, to the valve body of the shutoff valve so that, upon energization of the electromagnet, the valve body and the diaphragm are moved simultaneously, or first the valve body is moved and thereafter the diaphragm. This avoids the disadvantage that, at the beginning of the suction stroke, an unnecessarily high vacuum is produced which could result in the formation of vapor bubbles in the fuel.
In order to be able to execute the delivery stroke of the diaphragm in dependence on the pressure difference alone, it is advantageous to disconnect the armature of the electromagnet mechanically from the diaphragm during the delivery stroke of the latter.
In a preferred embodiment, the armature is disposed to be longitudinally movable in a pipe surrounded by the coil of the electromagnet, this pipe having at one end a valve seat for the valve body of the shutoff valve connected to the armature and terminating at the other end in the second chamber of the diaphragm pump and receiving a plunger arranged with an axial play between the diaphragm and the valve body, this play being larger, when the electromagnet is not energized, than the delivery stroke of the diaphragm. Thus, the diaphragm can execute its delivery stroke without coming into contact with the armature. If the armature is energized, it first lifts the valve body of the shutoff valve off its seat before it can lift the plunger and move the diaphragm to execute its suction stroke.
In vehicle heating units, special difficulties arise with respect to an exact metering of the fuel to be admixed to the conveyed amount of combustion air, since often influential variables are present which change considerably, and which must be taken into account. In this connection, the difficulty occurs that the throughflow characteristics of a venturi tube and of a fuel nozzle, on the basis of the differing flow conditions (one operating in the turbulent zone and the other in the laminar flow zone), do not have a similar curve formation. Thereby, only a relatively narrow region exists within which an ignitable fuel-air mixture is present. Since in case of automotive vehicle heating systems, the combustion of air fan is usually operated by the vehicle battery, the voltage of which is subject to considerable fluctuations during operation, resulting in corresponding variations of the conveyed amount of combustion air, it is advantageous to provide for a control of the differential pressure determining the delivery stroke of the diaphragm pump which is dependent on the amount of air. This can be accomplished, for example, by arranging a spring-loaded displacement member in the venturi, this member changing, with an increasing amount of air and a corresponding back pressure, the cross section of the venturi so that the throughflow characteristics of the venturi and of the fuel nozzle approach each other. Another possibility resides in a variation of the pressure, dependent on the amount of air, in the chamber connected to the air feed line outside of the venturi tube, by means of a controllable blowoff or bleeder valve. Another way to make the throughflow characteristics of the venturi tube and the fuel nozzle approach each other consists in the provision of a spring effective on the diaphragm in the direction of the delivery stroke. Thereby, a change is obtained in the differential pressure at the fuel nozzle, whereby the throughflow quantity of the fuel nozzle is then adapted to the air throughout of the venturi tube within a broad range. Also, frictional and resistance forces of the diaphragm can be compensated for by the spring.
The temperature exerts the greatest influence on the conveyance of the fuel in dependence on the amount of air, since the viscosity of the fuel is temperature-dependent and since the throughflow through the fuel nozzle is, in turn, dependent on the viscosity. Since a vehicle heating unit must operate satisfactorily within a very wide temperature span, the elimination of temperature effects is of special importance. This can be achieved, for example, by providing that the spring effective on the diaphragm in the direction of the delivery stroke is made of a thermal bimetal and is arranged preferably within the fuel stream taken in by the pump. Another correction possibility resides in making the cross section of the fuel nozzle variable in dependence on the fuel temperature, along the lines that the cross section is reduced with an increasing temperature and enlarged with a decreasing temperature. Under practical conditions, this can be accomplished by means of a nozzle having a conical nozzle needle which is displaced, for example, with the aid of an element made of a stretchable material. The same purpose is attained by a temperature-dependent variation of the nozzle length so that this length is reduced with an increase in the temperature. Finally, a temperature-dependent change of the differential pressure between the chambers of the metering pump could also be provided, so that in case of low fuel temperatures a higher differential pressure is ambient than in case of higher temperatures. This could be achieved in practice by means of a blowoff valve connected with the first chamber and operated by an element of a stretchable material.
In preferred embodiments of the invention, the coil of the electromagnet can be arranged on one side and the shutoff valve can be disposed on the other side of the diaphragm, the armature being extended through the diaphragm with the aid of a gasket or other sealing means.
Alternatively, the diaphragm can be equipped with a permanent magnet opposed to one end of the electromagnet, and the shutoff valve can be equipped with a permanent magnet opposed to the other end of the electromagnet, wherein the mutually facing ends of the energized electromagnet and of the permanent magnets have the same polarity. Upon excitation of the electromagnet, the permanent magnets are repelled whereby, on the one hand, the diaphragm is moved in the direction of its suction stroke and, on the other hand, the shutoff valve is moved in the direction toward its open condition.
In each of the above-noted preferred embodiments, a switch is associated with the electric lead to the electromagnet, this switch being operable by the diaphragm.
Additional details and features of the invention can be seen from the following description in conjunction with the drawings wherein several embodiments of the invention are shown in principle.